(1) Field of the Invention
The present invention relates generally to package sorting conveyors and more particularly relates to a sorting conveyor having an improved linear induction motor.
(2) Description of the Related Art
Conveyor systems having a number of individual carrying carts have been commonly used for many years to carry and sort packages or other items, such as mail. For example, U.S Pat. No. 5,054,601 to Sjogren et al. discloses a package sorting conveyor comprised of a train of tilt tray carriers coupled in tandem to form a continuous loop. Each carrier includes a pivotally mounted tilt tray normally maintained in an upright position. The carriers are moved around the loop by a series of motors spaced around the loop. Branching out from the loop are outfeed chutes or the like for receiving packages from the carriers. When a particular carrier holding a particular package to be sorted reaches a selected outfeed chute, an actuator tilts the tray to dump the package into the outfeed chute. Another example of a typical package sorting conveyor is disclosed in International PCT Application Number PCT/DK90/00047 of Kosan Crisplant A/S, now U.S Pat. No. 5,664,660.
One significant disadvantage of conventionally designed package sorting conveyors is that conventional conveyor carriers laterally tilt only on a horizontal axis parallel to the direction of conveyor travel. While this accomplishes the objective of dumping the package from the carrier into an outfeed chute or the like, the package is often roughly tumbled or rolled, sometimes damaging the package's contents. One reason for this is that the packages typically are unloaded from the carrier while still traveling forward at the same speed as the conveyor. Thus, packages tend to slam into a forward retaining wall of the outfeed chute before sliding down the chute. Another problem with conventional laterally tilting conveyors is that because the packages are moving forward at full speed when they are unloaded into the outfeed chute, the outfeed chute must be relatively wide so that packages do not miss the chute and fall off the conveyor past the chute. This often unnecessarily increases the overall size of the conveyor system.
U.S Pat. No. 4,744,454 and an improvement thereto, U.S Pat. No. 5,086,905, both to Polling, disclose previous attempts to remedy this problem of rough handling by conventional laterally tilting conveyor carriers. Both of these patents to Polling disclose a conveyor element for a package conveyor that includes a tilting carrier tray mounted to be rotatable about two swivel axes. A first swivel shaft extends obliquely downward from the underside of the carrying tray and is in turn connected at an angle to the end of a second swivel shaft extending obliquely upwards from a base support part of the conveyor element. Together, the two swivel shafts form a "V" that points in the direction of conveyor travel. Both of the swivel shafts lie in the vertical plane of symmetry of the conveyor element when the carrier tray is disposed in its upright position.
Because the carrier tray of Polling rotates about two oblique axes, the carrier tray can be tilted not only lateral on a horizontal axis, but is moved through a geometrically complex spatial reorientation during package discharge. This allows for more gentle placement of a package on an outfeed chute than can be accomplished using conventional conveyor trays that laterally tip on only a horizontal axis. The Polling conveyor element more gently handles the packages by imparting some degree of rearward velocity to the packages as they are discharged, which, when added to the forward velocity of the conveyor system, results in the packages'forward velocity during discharge being less than that of the conveyor system itself.
However, the conveyor elements of both of Polling's patents are unduly complicated and intolerant of manufacturing discrepancies. In fact, the second Polling conveyor element (U.S Pat. No. 5,086,905) was invented in an attempt to simplify the original design disclosed in the first Polling patent (U.S Pat. No. 4,744,454), which had proved to be too expensive and complicated to manufacture efficiently. As a result of this complexity and cost, the Polling devices have not enjoyed significant commercial acceptance and success.
One solution to these problems is shown in U.S. Pat. No. 5,836,436, issued Nov. 17, 1998 and co-owned by the assignee of the present invention, which is hereby incorporated by reference in its entirety. One feature of this reference is the use of a pull-down design for tilting the improve trays.
Another example of a sorter conveyor using a pull-down design for tilting its trays is shown in U.S. Pat. No. 5,664,660, issued to Prydtz. The '660 patent is directed to a sorter conveyor having laterally tiltable transport trays, instead of a conventional transport chain driven by a driving station, in which a driving wheel or worm cooperates with the chain links, these being advanced arranged with guiding rails along a small mutual spacing and carrying both the transport trays and the tilt mechanisms associated therewith, which conventional arrangement causes considerable problems with respect to tolerances for both the longitudinal pitch of the chain links and the mounting of the guiding rails. The invention includes carts that are advanced along mutually widely spaced guiding rails, whereby the tolerance demands on the latter are reduced considerably. Also, the carts are advanced by way of stationary linear motors, which, supposedly, renders any tolerance demands on the longitudinal pitch of the tray units entirely superfluous. Also the tilting mechanisms may be given a very simple design according to the patent. However, it has been discovered that there are operational problems associated with this type of design, especially the operation of its linear motors and tilting mechanism.
While the design shown in U.S Pat. No. 5,836,436 overcomes some of the problems associated with the prior art designs, some problems have begun to surface over time as the units have been operated. One of these problems is when there is a mis-tip which causes "cascading errors." "Cascading errors" were first discovered when audits at the end of the chute indicated there were more errors occurring than were predicted by errors indicating by tray verification. Normally, when a tray is to tip to the right immediately before the tray is being straightened, a series of photocells determines if the right hand arm is in the "low" or "tipped" position, or the "up" or "latched" position. If the controls said that the arm should be tipped and the verifier says that the arm is tipped then it was assumed that the product went down the correct chute. However, in certain cases, tray verification would predict five errors but chute audits would indicate 35 errors. This previously unknown effect is referred to in this application as "cascade errors."
Most pull-down tilting mechanisms are actuated by spring when the pivot switch is released by an energizing solenoid. In other words, the pivot switch is spring-biased in the "out" position but held in the retained position by the solenoid.
When the computer provides a pulse, the solenoid plunger retracts, enabling the pivot switch to spring into the "out" position. The wheel strikes the pivot switch in front of the pivot point and the pivot switch unlatches the carriage and then as the wheel strikes the tail of the pivot switch, it pushes the pivot switch back to the home position and the spring on the solenoid plunger pushes the plunger into a detent capturing the pivot switch in the home position. The computer initiates the unlatch but it is the action of the wheel on the tail that normally relatches it.
If the solenoid does not return to capture the pivot switch before the wheel leaves the pivot switch's tail or a wheel is missing or broken, this will allow the pivot switch to move away from its intended home position back into the engagement position. The next tilt module that is not in the down position will then strike the pivot switch, tilt, and reset the pivot switch. If the pivot switch relatches properly this time, then that particular pivot switch will cause no more errors. If the pivot switch does not relatch it can again go into the engaged position and cause additional problems.
The cascade effect, though, comes from the first carriage that was tipped incorrectly, for if it were assigned a tilt position further downstream, when that carriage (which has been tilted incorrectly) gets in position to unload, the pivot switch at the proper destination chute will move into the engagement position. But because that arm has been incorrectly tipped upstream, no action will take place and the pivot switch will stay in the out position until the next untipped carriage arrives. That carriage will then be tipped at the wrong destination but should cause that pivot switch to relatch. If that particular tray was assigned a chute destination further downstream, when it arrives at its proper destination, the pivot switch will move into the engagement position, but again cannot engage because the tilt module is already in the low position, the tilt module will pass by and will not relatch the pivot switch. This pivot switch will then stay in the out position capturing the wheel of the next tilt module that is not tipped. This error will than continue to cascade downstream until eventually the error will pass the last destination and the system should heal itself.
However, there are some conditions that can exist that cause the error not to heal itself but continue to cause problems. An example would be if a particular tilt module were missing a wheel; that tilt module could then be loaded and when it gets to its destination, the pivot switch will go out to engage the wheel but with no wheel the module will not tip. The pivot switch will then catch the next available tray beginning the cascade problem. This will continue to repeat itself and will occur and start cascade error any time the tray with the missing wheel is assigned a destination.
One attempted solution to this problem is to use the tip verification switch to shut the sorter down if the pivot switch does not return to the "home" position within a given time period. However, with a large sorter this requires a maintenance man to then check the pivot switch to see if it should be reset and whether a wheel is missing. In the meantime, everything just sits on the sorter. For time critical applications, such as airline baggage sorting, this delay may be unacceptable.
One solution to these problems is shown in U.S. Pat. No. 6,009,992, issued Jan. 4, 2000 and also owned by the assignee of the present invention, which is hereby incorporated by reference in its entirety. One feature of this reference is the use of a powered up and powered down bi-directional pivot switch.
However, most of these designs are based around linear induction motors (LIMs) which are well accepted today due to their quietness and low maintenance. Unfortunately, for all their advantages LIMs are not very energy efficient. For example, pairs of LIMs, spaced at 80 feet intervals and operated at 480 volts, draws about 15 amps. This is about 90 watts per foot of sorter for about 70 pounds of thrust. To the contrary, prior art chain drives would draw about 15 watts per foot of sorter for about 1500 pounds of thrust. Thus, the modern LIM drives, for all their other advantages, are energy hogs. However, prior art, mechanical chain drives are just too noisy and high maintenance for today's market.
Thus, there remains a need for a new and improved LIM sorting conveyor that is substantially more energy efficient while, at the same time, is mechanically as quiet and maintenance-free as the modern LIM drives.